Forest Ecology Basics
- Shade Tolerance – is the ability of a tree species to be photosynthetically active at certain levels of sunlight. Shade tolerance is measured by a tree’s capacity to survive low overhead light. For example, hemlock, sugar maple, and beech are very shade tolerant. Red maple, oak, and white pine are intermediate in shade tolerance. White birch and poplar are very intolerant to shade.
- Forest Succession – shade intolerant tree species are replaced by more shade intolerant tree species over time. For example, a forest that starts out as white birch and poplar will eventually be replaced by oak and white pine which in turn may be replaced by hemlock, sugar maple, and beech over long periods of time.
- Growth and Competition – since space and resources (light, nutrients, and water) are limited in any forest, competition between individual trees is always present. It increases from little or none in the early stages of forest development to intense in the later stages. For example, a young stand of trees may start with 4,000 – 6,000 seedlings per acre. At maturity, when they measure at least 20 inches in diameter, about 100 trees per acre will survive. So as a forest grows and develops, a lot of trees lose the race for survival. This is a natural selection process and it is slow sometimes taking 150 years or more to complete. Thinnings hasten this process and promote the more desirable trees to achieve maturity in a reduced period of time – often less than 100 years.
Forestry is the profession embracing the science, art, and practice of creating, managing, using, and conserving forests for human benefit in a sustainable manner to meet the desired goals of ownership.
The role of foresters is to help develop policies about the objectives of forest management and to execute them through the formulation of silvicultural prescriptions. On a private woodlot, for example, foresters analyze the natural and social factors bearing on each forest stand and then devise and conduct treatments most appropriate to create and maintain the kind of forest that will best fulfill the goals of landowners or public stakeholders. In Massachusetts, the Forester Licensing Law requires that these functions be carried out by Licensed Foresters to protect the public interest.
The Massachusetts Bureau of Forestry reviews Forest Cutting Plans, Chapter 61 Forest Management Plans, and Forest Stewardship Plans to determine whether they will promote good forestry or if they call for destructive practices such as high-grading or liquidation cutting.
What is Silviculture?
Silviculture is the art and science of producing and tending a forest. It includes controlling the establishment, growth, composition, health, and quality of forests to meet the diverse needs and values of landowners and society on a sustainable basis. The ideal goal of silviculture is to place the right tree in the right place with just the right amount of growing space at each stage of development. The first consideration in determining which species to grow is the degree of their adaptability to a site. In terms of timber production, it is often better to grow species that exhibit superior growth on a given site. Site index is a measure of forest productivity and is used as an indicator of site quality. It is used for different species and can be looked up in your local Soil Survey Map published by the USDA Natural Resources Conservation Service. Site index is the average height of the dominant and codominant trees of a species in even-aged forest stands and is usually measured at age 50. It is a good measure of how productive a site and species is because height growth is considered to be independent of stand density. Site indexes are significantly higher for red oak and white pine than they are for hemlock and red maple.
There are two categories of silvicultural treatments: Intermediate Cuttings and Regeneration Cuttings. Intermediate cuttings are thinnings or improvement cuttings which adjust stand density to optimize forest growth and control forest composition by promoting more desirable species such as red oak over red maple. Thinnings are needed in forest stands that are too dense while improvement cuttings are often needed to rehabilitate forest land that has been subjected to destructive logging called high-grading. These cuttings will increase the growth rate (as much as two times!) and vigor of the more desirable trees. Regeneration cuttings are used in mature forest stands to reproduce a new forest.
A timber cruise is a forest survey which is done to locate and estimate the quantity of timber on a given property according to species, size, quality, possible forest products, and other characteristics such as wildlife habitat. Information tabulated from the timber cruise is used to draw forest type maps, develop silvicultural prescriptions, and write forest management plans.
It may not be in the interest of two different landowners to manage the same stand in the same way. If an owner is most interested in wildlife or preserving some old-growth timber, for example, then a forester can modify the silviculture accordingly. However, this is never an excuse to liquidate a forest because there is no silviculture in a liquidation cutting (high-grading)!
Thinning Forest Stands
Thinning forest stands correctly is one of the most important jobs for a forester. Thinning is primarily intended to optimize economic returns for timber production. A program of thinnings is essentially a series of temporary reductions made in stand density to maximize the net value of timber products removed during the whole rotation. Although thinning can certainly be used for other objectives of managing forest vegetation, timber production is the chief purpose.
Thinning improves the economic yield of forest stands by:
- Salvage of anticipated losses of merchantable volume from natural suppression.
- Increase in value from improved diameter growth of the higher quality and higher value trees like red oak and sugar maple. Timber value is enhanced by favoring the trees of best quality and highest market value (such as red oak, sugar maple, white pine, and cherry) and discriminating against the lower quality and lower value ones (such as red maple, beech, and hemlock). Although disease free beech, some hemlock, and trees with cavities (den trees) are often retained for wildlife purposes.
- The positive effect of thinning on wood quality is the most important reason for thinning. So it is usually very important to allocate growing space to the trees that are growing in value the most.
- Thinning can control species composition (the kinds of trees) and effect future forest regeneration. It can reduce the seed source of undesirable trees such as red maple, while increasing vigor, mechanical strength, and seed production of the more desirable trees such as red oak.
- Another positive effect of thinning is that it increases the vigor of trees by giving each tree more growing space. Increased vigor increases the tree’s ability to respond to most insect attacks and some tree diseases. Thinning also leads to reduced height/diameter ratios and greater wind firmness (although this is sometimes not the case in forested wetlands where thinning may increase windthrow hazard of the residual stand.)
This chart shows the dramatic difference a thinning can make in diameter growth of selected crop trees. A free-to-grow rating of 0 means that a tree has no room to grow. A rating of 4 means that a tree is free to grow on all 4 sides which can more than double its growth rate.
Thinning Oak, Hardwood Stands in New England
The primary species in the oak, hardwood forest type are red oak, red maple, black birch, black oak, white oak, hickory, and other hardwoods with red oak being the most valuable and important.
The objective in oak, hardwood stands is to produce a long branchless bole (trunk) in order to produce high quality and high value sawlogs. This is accomplished by growing trees at high densities when they are young so that competitive pressure will encourage self-pruning. So thinning can be delayed until the oak stand is about 45 years old or when the trees average between 6-10 inches in diameter. Trees of this size, commonly referred to as poles, respond rapidly to thinning. At this age, cordwood can be harvested in the first thinning. The trees selected as crop trees will be straight and the lower 10-16 feet should have few or no branches. The crowns (the upper part of the tree bearing live branches and foliage) of the crop trees need at least 3 or 4 feet of open growing space on at least two sides. Low quality competing trees that are touching the crowns of the crop trees should be cut. After the initial thinning, additional thinnings can be made every 10 years or so. Each time a stand is thinned, enough trees, as uniformly spaced as possible, must be left to fully utilize the site. The trees with good form, good vigor, and well-developed crowns should be favored. Thinning must be heavy enough to permit rapid growth of the high value red oak crop trees that remain, but not so heavy that epicormic branches are stimulated on the trunks of the crop trees which will greatly reduce their sawlog value.
Thinning Northern Hardwood Stands in New England
The primary species in the northern hardwood forest type is sugar maple. Other common trees are yellow birch, beech, hemlock, ash, and cherry.
As in the oak type, a commercial thinning is often delayed until the stand is 40-50 years of age (6-10 inches in diameter) when trees can be cut for cordwood. From that time until the final harvest, periodic thinnings will provide income for the landowner while improving the growth and quality of the remaining trees. In the first thinning, about 75-100 crop trees/acre are picked and the lower quality and lower value competing trees are cut around them. The crop trees should be tall, straight trees with sizable live crowns exposed to direct sunlight. These trees have the greatest potential to grow rapidly into high value trees.
Thinning White Pine Stands in New England:
Thinning white pine stands should begin as soon as possible when the trees average at least 6 inches or more in diameter. The objective is to select the maximum number of straight high quality crop trees and cut the low quality competitors. In general, pines less than 30 years old with at least 1/3 of their height in live crown respond the best, while response declines with increasing age and decreasing crown length. Pure stands of white pine often never stagnate. Because of variations in inherent vigor, differentiation into crown and diameter classes almost always occurs. Therefore, one of the best ways to thin a white pine stand is from below. This means that most of the trees removed will be from the lower crown classes. This type of thinning accelerates the natural mortality of a forest stand as it develops over a long period of time by removing those undesirable trees that are of lower quality and are losing the race for survival. This type of thinning is also commonly used in other forest types as well. Periodic thinnings are guided by using a stocking table which gives the desired number of trees per acre for a given average stand diameter. Stocking guides are used for other forest types as well.
It should be noted that thinnings are often modified for mixed stands (different species composition) and in forest stands that have been high-graded in the past.
Regenerating Forest Stands
Regenerating Oak, Hardwood Stands and Northern Hardwood Stands
The selection system and the shelterwood system are often used to regenerate mature oak stands. In the selection system, scattered individual or small groups are harvested at periodic intervals every 10 to 20 years. Trees of all sizes are cut and remove no more than 1/3 of the trees. Trees with good crowns and vigor are favored. It is possible to regulate the amount cut so that it equal’s the forest’s growth. That way you can make periodic selection cuts indefinitely. Selection cutting is often the best option because it is good for: small woodlots; provides more frequent periodic income; maintains a range of tree sizes; has little negative visual impact; and log quality is often higher using this system.
In the shelterwood system, the overstory is removed in two or three cuttings over a period of 10-20 years. The first cutting removes at least 1/3 of the lower quality trees and leaves the higher quality trees to put on additional increment (diameter growth) while providing “shelter” to the developing seedling regeneration. When enough vigorous young trees are established, the remaining overstory is harvested so the young trees will have space to grow rapidly. Shelterwood cutting can provide for a greater variety of species but the periods between income are much longer and the negative visual impacts last longer. A modification of the shelterwood system would be to leave some of the larger, healthier, and fully crowned trees as “standards” for aesthetic purposes which may or may not be harvested in the future. This would mitigate some of the adverse visual impacts of the final harvest.
Regenerating White Pine Stands
The shelterwood system is commonly used for white pine because the overstory pine protect the young white pine saplings from damage caused by the white pine weevil which is an insect that kills the leaders of white pine trees decreasing their value. The two or three cut shelterwood system can be used. Since the overstory pine can remain for a long time (10-20 years), it should be composed of the highest quality pines that can produce value rapidly to compensate for the initial slow growth of the white pine seedling regeneration in the understory. The best overstory trees should be well distributed throughout the stand. Openings should be kept smaller than 1/2 the tree length. The first cut should ideally be made during or right after an abundant seed year (which occur every 3-5 years) and consist of removing 40-60% of the overstory. It is essential that the first cut, preferably in the snowless months, results in the disturbance of accumulated litter and exposure of mineral soil so that white pine seed will have a better chance to germinate and grow. The second or third cut to remove the overstory shelter trees can take place after the young pines have entered the period of rapid growth or are at least one log length tall (16 feet). Here again, the system can be modified to leave some big healthy trees as “standards” for aesthetic purposes especially on the smaller woodlots.
This discussion on thinning and regenerating forest stands gives the average person an idea of what good forestry is all about. By practicing good forestry, you can improve the value of your forest and your property and continue to enjoy the many benefits it provides such as timber production, wildlife habitat, clean air and water, and recreation.
Silvics of Forest Trees of the United States – Agricultural Handbook No. 271
The Practice of Silviculture. David Smith, Bruce Larson, Matthew Kelty, and P. Mark Ashton. 1997.
Choices in Silviculture for American Forests. Society of American Foresters. 1981.
A Management Guide for Oak in New England. David Hibbs and William Bentley.
Connecticut Cooperative Extension Service.
Managing Northern Hardwood Stands. Craig Lorimer and Craig Locey.
Wisconsin Cooperative Extension Service.
A Silvicultural Guide for White Pine in the Northeast. Kenneth Lancaster and William Leak.
US Forest Service General Technical Report NE-41.
How to Manage White Pine. Jeff Martin and Craig Lorimer.
Department of Forestry, University of Wisconsin.
Improve Your Woodlot by Cutting Firewood.
USDA Forest Service Northeastern Area.
This is a high quality white pine forest stand marked for a thinning on a 100 acre woodlot in Hardwick, MA.
White pine is our largest conifer in the northeast and one of the most important commercial timber species here. It is used for construction, furniture, and pulpwood.
This is an oak, hardwoods forest stand marked for a thinning. The high quality red oak tree in the foreground
will benefit by removing the marked low value red maple tree next to it on the right.
This is a group of white ash trees marked for a thinning. This is called “thinning from below” which means that
most of the trees removed will be from the lower crown classes. This type of thinning accelerates the natural mortality of a forest stand as it develops over time by removing those declining trees that are of lower quality and are losing the race for survival due to competition from other trees.
A high quality yellow birch tree is in the foreground in a well managed hemlock, hardwoods forest stand in Petersham, MA.
These are large sugar maple trees next to a stream in Petersham, MA. Sugar maples are well known for producing maple syrup and for their beautiful fall foliage, but it is also one of our most valuable timber trees and its wood is used for furniture, flooring, and veneer.